Many types of electronic measuring instruments, such as oscilloscopes and counters, require an input trigger signal to begin their measuring sequence. Normally, the trigger signal is generated by detecting the voltage at which an AC signal to be measured crosses a reference level by means of a high gain amplifier. Since all signals have some superimposed noise, it is necessary to set the reference level high enough to prevent false trigger signals caused by the noise at the input to the high gain amplifier. Conventionally, this is accomplished by means of a Schmitt trigger type circuit which provides positive feedback, or hysteresis, in the high gain trigger signal generating amplifier. Such hysteresis helps to reduce the chance of multiple trigger signals as the noisy input signal crosses the reference level on the input of the trigger amplifier. If the trigger level is too low, the instrument may experience false triggers caused by the noise resulting in alternate slope triggering with the displayed signal flickering or the counter displaying various results.
However, the amount of hysteresis needed to prevent false triggering in the presence of noise usually requires a manual adjustment by the operator or during calibration so that the hysteresis need not be set so high as to miss the actual reference level crossing by the AC input signal.
Unfortunately, since the trigger hysteresis adjustment is done manually it is both an operator inconvenience and a potential source of expensive recalibration. It would be desirable, to have a trigger circuit which could automatically adjust its own hysteresis sensitivity as the signal level to be measured changes with low sensitivity for large signals and high sensitivity for small signals.